After toe off there is the propulsive stretch reflex of the ankle complex that helps initiate the recovery action of the leg, when the foot and leg is behind the body and fully extended this results in flexion of the hip and knee to an extent, this results in foot obviously moving towards front side of body but knee flexion also brings foot upwards towards butt rapidly, this action varies in height with different athletes (Usain Bolt got his foot way up there, Asafa Powell much less) but almost all elite sprinters see the heel coming up towards the butt or very near it, bottom of shoe facing pretty close to upwards or in that vicinity. After the hip rotates forward, knee that is flexed starts to open up forward into roughly that 90 degree front side shape, and then subsequently extends downwards at knee and hip joints. There is this particular action of the knee extensors/flexors where the leg is closed like a jackknife behind the body with heel near butt, and right when the leg crosses underneath the hips it seems to balistically open and extend on the frontside. Even though this involves the muscle shortening first, and then lengthening how significant a contribution does the SSC play into this action of the knee? Seems like the fastest athletes get this huge whip effect during this “buttkick”/backside mechanics tendency?
That is quite an analysis SeaCashew.
I am not sure what you are asking?
The super cool part of sprinting is how to balance the enormous strength and power aspect with the flexibility of all joints and limbs.
If you are looking to understand more in-depth speed mechanics I strongly suggest you watch any of Charlie’s lectures.
Lots of people want to reinvent the wheel with regards to coaching or training but learning from the top coaches needs to be part of the learning curve now or soon for anyone with more questions than answers and RESULTS.
Sorry, this is near impossible to describe without videos and pictures, the limits of human communication 
I am questioning the importance of the stretch reflex of knee flexors and extensors during the recovery action of the leg, specifically when the foot is right near the butt and switching to the frontside of the body.
Watch a slow motion of Bolt (or Blake, Gay, Gatlin) and watch the rear leg being recovered. After toe-off the leg bends at knee, heel comes up almost touching butt (pic 1), hip rotates leg to front of body (pic 2), and knee extends automatically on front side reminiscent of stretch reflex, its automatic and ballistic reaction to knee closing quickly). I have analyzed a bunch of fast athletes from HS to elite doing this, seems like all the fast athletes do this significantly, and even though leg takes a longer range of motion to recovery it is necessary to do because the stretch reflex makes a larger vertical impulse? This is almost a necessary backside mechanic, I noticed a deficit of this in my own running and I knew something was up with my stride length, but my stride frequency was much faster. Should athletes aim for this consciously? Seems like I was taught to drill away any and all backside mechanics, but this is not talked about and is massively impactful.
What a thorough analysis. SeaCashew.
What question are you asking, exactly?
How to blend the great strength and power factor with the flexibility of all joints and limbs is the most awesome feature of sprinting.
I highly recommend watching any of Charlie’s lectures if you want to comprehend speed mechanics in greater detail.
Many people want to recreate the wheel when it comes to coaching or training, but anyone who wants results and has more questions than answers needs to start learning from the best coaches as soon as possible.
I a m questioning the importance of the stretch reflex of knee flexors and extensors…
Hip, knee, ankle extensors /flexors all have important roles. If you have a weakness at one joint then you will lose power in that particular phase, You need powerful knee extensors to absorb landing then propel foward movement. The flexors recovery the leg and then accelerate back under hips.
Stretch reflex energy comes from all joints. You cant break down one segment and conclude its zero or low contribution.
Most of the recoil comes from the ankle joint but hip/ knee also important.
Stretch reflex at knee joint when the recovery leg is crossing from backside to front side after the butt kick action. You can absolutely break down one segment and conclude it is also of the contribution in this case because it is the only segment experience load or resistance (in the form of inertia). Since its been months since I made this post I learned that this action - *again the transition from when the recovering heel is by the butt, switching to the frontside of the body rapidly and utilizing high heel recovery to increase stride length/stride frequency also relies on conservation of angular momentum increasing velocity since the leg is folded and the R distance shortens so recovery velocity increases.